Magnetic separation is a recent simple technique for isolation of cells, particles and organic molecules from complex mixtures by association, conjugating or labeling the molecule desired with a magnetic responsive material. The magnetic responsive material can be microbeads, such as the superparamagnetic beads available from Dynal Biotech as Dynabeads, magnetic nanoparticles, such as StemSep available from StemCell Technologies, and other magnetic particles or molecules that can be combined or attached to the cell or organic molecule of interest. Such magnetically “labeled” organic material is then positioned within a magnetic field to effect separation of the labeled material. The advantage of magnetic separation is that the process is gentle, and hence does not present the potential physical and/or chemical damage that may result with centrifuge separation methods. Selection can be positive (to isolate and retain the magnetic labeled particles) or negative (to remove or exclude the magnetic labeled particles)
Magnetism can also be used to stir or mix materials. However, in prior art magnetic separation/stirring technologies, fixed bulky bar magnets are used. For instance, prior art magnetic separators employ bar magnets. The sample containing the labeled magnetic material is positioned adjacent to the magnet, and left for a period of time to allow labeled particles to migrate to the magnet under the influence of the magnetic force. A magnetic separator manufactured by Dynal Biotech employed a single bar magnet and the samples are placed adjacent to the magnet for separation. Another magnetic separator manufactured by Invitrogen (the Captivate Microscope Mounted Magnetic Yoke) uses two bar magnets mounted horizontally side by side with a center horizontal channel between the magnets. A sample containing a ferrofluid is positioned in the channel, where separation occurs though the magnetic forces exerted by the bar magnets. The yoke is designed to be inserted into a compound microscope where the process can be monitored. Another magnetic separator available from Miltenyi Biotec is similar, but uses a vertical yoke having two bar magnets positioned vertically side by side with a channel or column positioned there between. The material is flowed into the column, where separation occurs.
Prior art magnetic mixers or stirrers generally utilize a mixing agitator or paddle positioned within a container. The paddle is rotated within the container through the use of externally generated magnetic fields, such as created by externally rotating magnets, such as taught in U.S. Pat. Nos. 5,478,148; 5,586,823 and 6,383,827. These prior art magnetic mixing tools are cumbersome and the strength of the magnetic field is difficult to modify and control without replacing the magnets. There is no ability to vary the application of the magnetic force spatially. These tools lack compactness and could not be used to move ferrofluids in vivo (for purposes of this application, a ferrofluid is a flowable substance where a portion of the substance is responsive to a magnetic field). Materials that are responsive to a magnetic field are referred to as M particles. Hence, a portion of a ferrofluid must consist of M particles. The ferrofluid may have nanoscale or micrometer sized M particles suspended in a carrier fluid, or cells incorporating a magnetically responsive material suspended in a carrier fluid. M particles, if contained in a carrier, may appear as a solid or a liquid if separated from the carrier.